2011年6月30日木曜日

Risk of collective failure provides an escape from the tragedy of the commons

Francisco C. Santos and Jorge M. Pacheco
PNAS June 28, 2011 vol. 108 no. 26 10421-10425

From group hunting to global warming, how to deal with collective action may be formulated in terms of a public goods game of cooperation. In most cases, contributions depend on the risk of future losses. Here, we introduce an evolutionary dynamics approach to a broad class of cooperation problems in which attempting to minimize future losses turns the risk of failure into a central issue in individual decisions. We find that decisions within small groups under high risk and stringent requirements to success significantly raise the chances of coordinating actions and escaping the tragedy of the commons. We also offer insights on the scale at which public goods problems of cooperation are best solved. Instead of large-scale endeavors involving most of the population, which as we argue, may be counterproductive to achieve cooperation, the joint combination of local agreements within groups that are small compared with the population at risk is prone to significantly raise the probability of success. In addition, our model predicts that, if one takes into consideration that groups of different sizes are interwoven in complex networks of contacts, the chances for global coordination in an overall cooperating state are further enhanced.

Trial by trial data analysis using computational models

Nathaniel D. Daw
Chapter for "affect, learning, and decision making: attention and performance xxiii"
August 27, 2009

In numerous and high-pro?le studies, researchers have recently begun to integrate computational models into the analysis of data from experiments on reward learning and decision making (Platt and Glimcher, 1999; O'Doherty et al., 2003; Sugrue et al., 2004; Barraclough et al., 2004; Samejima et al., 2005; Daw et al., 2006; Li et al., 2006; Frank et al., 2007; Tom et al., 2007; Kable and Glimcher, 2007; Lohrenz et al., 2007; Schonberg et al., 2007; Wittmann et al., 2008; Hare et al., 2008; Hampton et al., 2008; Plassmann et al., 2008). As these techniques are spreading rapidly, but have been developed and documented somewhat sporadically alongside the studies themselves, the present review aims to clarify the toolbox (see also O’Doherty et al., 2007). In particular, we discuss the rationale for these methods and the questions they are suited to address. We then offer a relatively practical tutorial about the basic statistical methods for their answer and how they can be applied to data analysis. The techniques are illustrated with ?ts of simple models to simulated datasets. Throughout, we ?ag interpretational and technical pitfalls of which we believe authors, reviewers, and readers should be aware. We focus on cataloging the particular, admittedly somewhat idiosyncratic, combination of techniques frequently used in this literature, but also on exposing these techniques as instances of a general set of tools that can be applied to analyze behavioral and neural data of many sorts. A number of other reviews (Daw and Doya, 2006; Dayan and Niv, 2008) have focused on the scientific conclusions that have been obtained with these methods, an issue we omit almost entirely here. There are also excellent books that cover statistical inference of this general sort with much greater generality, formal precision, and detail (MacKay, 2003; Gelman et al., 2004; Bishop, 2006; Gelman and Hill, 2007).

2011年6月28日火曜日

Distributed Neural Representation of Expected Value

The Journal of Neuroscience, May 11, 2005  25(19):4806 - 4812
Brian Knutson, Jonathan Taylor, Matthew Kaufman, Richard Peterson and Gary Glover

Anticipated reward magnitude and probability comprise dual components of expected value (EV), a cornerstone of economic and psychological theory. However, the neural mechanisms that compute EV have not been characterized. Using event-related functional
magnetic resonance imaging, we examined neural activation as subjects anticipated monetary gains and losses that varied in magnitude and probability. Group analyses indicated that, although the subcortical nucleus accumbens (NAcc) activated proportional to anticipated gain magnitude, the cortical mesial prefrontal cortex (MPFC) additionally activated according to anticipated gain probability. Individual difference analyses indicated that, although NAcc activation correlated with self-reported positive arousal, MPFC activation correlated with probability estimates. These findings suggest that mesolimbic brain regions support the computation of EV in an ascending and distributed manner: whereas subcortical regions represent an affective component, cortical regions also represent a probabilistic component, and, furthermore, may integrate the two.

2011年6月27日月曜日

Neuronal basis of sequential foraging decisions in a patchy environment

Benjamin Y Hayden, John M Pearson & Michael L Platt
Nature Neuroscience 14, 933–939 (2011)


Deciding when to leave a depleting resource to exploit another is a fundamental problem for all decision makers. The neuronal mechanisms mediating patch-leaving decisions remain unknown. We found that neurons in primate (Macaca mulatta) dorsal anterior cingulate cortex, an area that is linked to reward monitoring and executive control, encode a decision variable signaling the relative value of leaving a depleting resource for a new one. Neurons fired during each sequential decision to stay in a patch and, for each travel time, these responses reached a fixed threshold for patch-leaving. Longer travel times reduced the gain of neural responses for choosing to stay in a patch and increased the firing rate threshold mandating patch-leaving. These modulations more closely matched behavioral decisions than any single task variable. These findings portend an understanding of the neural basis of foraging decisions and endorse the unification of theoretical and experimental work in ecology and neuroscience.

2011年6月22日水曜日

Neurobiology of Value Integration: When Value Impacts Valuation

Soyoung Q Park, Thorsten Kahnt, Jörg Rieskamp, and Hauke R. Heekeren
The Journal of Neuroscience, 22 June 2011, 31(25): 9307-9314


お金と電気ショックの効用をどう統合しているのか?独立に加算?交互作用ある?選択モデルを行動データにフィットしても判定できなかったが、前頭前野(報酬関連部位)の神経活動(fMRI)にフィットしたら「交互作用を含んだモデル」が支持された。

選択(意思決定)モデルをfMRI信号にフィットして「モデル比較」をするという発想は面白いけど、モデル内の変数(効用、価値とか)が神経活動レベルでどう表象されているのか分からない以上、難しいと思う。


Everyday choice options have advantages (positive values) and disadvantages (negative values) that need to be integrated into an overall subjective value. For decades, economic models have assumed that when a person evaluates a choice option, different values contribute independently to the overall subjective value of the option. However, human choice behavior often violates this assumption, suggesting interactions between values. To investigate how qualitatively different advantages and disadvantages are integrated into an overall subjective value, we measured the brain activity of human subjects using fMRI while they were accepting or rejecting choice options that were combinations of monetary reward and physical pain. We compared different subjective value models on behavioral and neural data. These models all made similar predictions of choice behavior, suggesting that behavioral data alone are not sufficient to uncover the underlying integration mechanism. Strikingly, a direct model comparison on brain data decisively demonstrated that interactive value integration (where values interact and affect overall valuation) predicts neural activity in value-sensitive brain regions significantly better than the independent mechanism. Furthermore, effective connectivity analyses revealed that value-dependent changes in valuation are associated with modulations in subgenual anterior cingulate cortex–amygdala coupling. These results provide novel insights into the neurobiological underpinnings of human decision making involving the integration of different values.

Frontal Cortex and Reward-Guided Learning and Decision-Making

Matthew F.S. Rushworth, MaryAnn P. Noonan, Erie D. Boorman, Mark E. Walton and Timothy E. Behrens
Neuron Volume 70, Issue 6, 23 June 2011, Pages 1054-1069

Reward-guided decision-making and learning depends on distributed neural circuits with many components. Here we focus on recent evidence that suggests four frontal lobe regions make distinct contributions to reward-guided learning and decision-making: the lateral orbitofrontal cortex, the ventromedial prefrontal cortex and adjacent medial orbitofrontal cortex, anterior cingulate cortex, and the anterior lateral prefrontal cortex. We attempt to identify common themes in experiments with human participants and with animal models, which suggest roles that the areas play in learning about reward associations, selecting reward goals, choosing actions to obtain reward, and monitoring the potential value of switching to alternative courses of action.

2011年6月15日水曜日

The Neural Correlates of Subjective Utility of Monetary Outcome and Probability Weight in Economic and in Motor Decision under Risk

Shih-Wei Wu, Mauricio R. Delgado, and Laurence T. Maloney
The Journal of Neuroscience, 15 June 2011, 31(24): 8822-8831

In decision under risk, people choose between lotteries that contain a list of potential outcomes paired with their probabilities of occurrence. We previously developed a method for translating such lotteries to mathematically equivalent “motor lotteries.” The probability of each outcome in a motor lottery is determined by the subject's noise in executing a movement. In this study, we used functional magnetic resonance imaging in humans to compare the neural correlates of monetary outcome and probability in classical lottery tasks in which information about probability was explicitly communicated to the subjects and in mathematically equivalent motor lottery tasks in which probability was implicit in the subjects' own motor noise. We found that activity in the medial prefrontal cortex (mPFC) and the posterior cingulate cortex quantitatively represent the subjective utility of monetary outcome in both tasks. For probability, we found that the mPFC significantly tracked the distortion of such information in both tasks. Specifically, activity in mPFC represents probability information but not the physical properties of the stimuli correlated with this information. Together, the results demonstrate that mPFC represents probability from two distinct forms of decision under risk.

2011年6月8日水曜日

Neural basis of conditional cooperation

Shinsuke Suzuki, Kazuhisa Niki, Syoken Fujisaki and Eizo Akiyama
Social Cognitive & Affective Neurosci
Volume 6, Issue 3 Pp. 338-347

自分の論文なので一応宣伝。

互恵的協力行動についてのfMRI実験。
参加者はfMRI中で囚人のジレンマゲーム(ランダム・マッチングの繰り返しゲーム)を行う。

行動データ:参加者は「基本的には協力するが、裏切り者(過去のゲームで裏切った回数の多い人)には裏切る」という傾向を持つ。
fMRIデータ:「裏切り者に対して裏切る」とき、dlPFC(前頭前野外背側部)が活動。
→ 互恵性(しっぺ返しやトリガー戦略)の神経科学的基盤


Cooperation among genetically unrelated individuals is a fundamental aspect of society, but it has been a longstanding puzzle in biological and social sciences. Recently, theoretical studies in biology and economics showed that conditional cooperation?cooperating only with those who have exhibited cooperative behavior?can spread over a society. Furthermore, experimental studies in psychology demonstrated that people are actually conditional cooperators. In this study, we used functional magnetic resonance imaging to investigate the neural system underlying conditional cooperation by scanning participants during interaction with cooperative, neutral and non-cooperative opponents in prisoner's dilemma games. The results showed that: (i) participants cooperated more frequently with both cooperative and neutral opponents than with non-cooperative opponents; and (ii) a brain area related to cognitive inhibition of pre-potent responses (right dorsolateral prefrontal cortex) showed greater activation, especially when participants confronted non-cooperative opponents. Consequently, we suggest that cognitive inhibition of the motivation to cooperate with non-cooperators drives the conditional behavior.